John Harris was ordered into the British Royal Navy in 1803, after a brief service in the merchant marine. During his time in the navy, Harris rose to the rank of Master where he was responsible for maintaining, outfitting and navigating the ship and was required to note features of coastlines that had not been recorded. Harris was ordered to assist with the survey of the Great Lakes, under Commodore Edward Owen, in 1814. One of his first assignment was to survey the north shore of Lake Erie for a ship building site. John Harris retired on half-pay from the Navy in 1817 and moved to a farm near Long Point with his wife, Amelia Harris.
Harris was appointed to Treasurer of the London District in 1821. As Treasurer, Harris was responsible for tax collecting, overseeing public expenditure, issuing and receiving receipts for the sale of land, and other financial matters for the London District. Following the Upper Canada Rebellion of 1837, Harris was appointed as a members of the Rebellion Losses Claims Commission. The Commission was established to review claims for losses suffered during the 1837 Rebellion in the London District and determine the compensation to be allotted.
John Harris remained active in the London political sphere until his death in 1850.